When you would play a deck with creature or weapon-based creature control (e.g. Otyugh, Parasite, Mindflayer), it is a real advantage to get your creature out 1 turn earlier with towers, especially, if you expect that the opponent has similar Creature Control. Or if you use growing creatures, which are weak in the beginning (e.g Forest Spirit, Lava Golem), the tower upgrade can be good.
Quantum Towers are also a good upgrade (although other upgrades in a rainbow deck are more important). It can win you games, if you are able to play a shield (or other important card) one turn earlier.
The upgrade of Nightfall, and the upgrade of the vampires helps you here to drain more life.